An electrophotographic image forming apparatus exposes a photosensitive member to a light beam (laser beam) emitted from a light source, such as semiconductor laser, to form an electrostatic latent image on the photosensitive member, and develops the image by using toner.
The light amount of a light beam depends on the temperature of a light source. In the case of semiconductor laser, for example, a rise in temperature of semiconductor laser causes light amount variation, i.e., reduction in amount of light emission for an identical drive current (degradation in droop characteristics).
Japanese Patent Application Laid-Open No. 2006-150601 discusses a method for driving a light emitting element described below as a technique for restricting light amount variation in a light beam emitted from a light source due to temperature variation. An image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2006-150601 drives a light emitting element not used for image forming, detects its operating state (for example, the amount of variation in drive current value having a temperature dependency), and controls the light amount of a light beam emitted from a light emitting element used for image forming based on the detected operating state. The technique discussed in Japanese Patent Application Laid-Open No. 2006-150601 enables correcting the light amount of a light beam varying with temperature change, without providing on the light emitting element a unit for detecting the temperature.
In an apparatus having a plurality of drive units for respectively driving different light emitting elements out of a plurality of light emitting elements, if each of the plurality of drive units drives a light emitting element based on the method for driving a light emitting element discussed in Japanese Patent Application Laid-Open No. 2006-150601, the following problem arises. Specifically, since there is an individual difference between circuit configurations of the plurality of drive units, each drive unit supplies a slightly different drive current to a light emitting element not used for image forming, based on a common setting value. Therefore, if each drive unit individually performs control as discussed in Japanese Patent Application Laid-Open No. 2006-150601, there arises a difference in light volume between light beams emitted from the plurality of light emitting elements used for image forming controlled by respective drive circuits.